There are two goals of the project that utilize mass spectrometry: 1) the metal binding site on the regulatory proteins of the plasmid-encoded ars operon will be determined by MALDI MS of the purified proteins with bound inducer. In preliminary experiments the dimeric ArsR repressor was shown to contain 1 mol Sb(III) mol ArsR monomer. 2) the ArsA ATPase is the catalytic subunit of the pump. It has been labeled with a covalent modifying reagent that contains As(III). The derivatized enzyme was digested with trypsin, and the tryptic fragments analyzed by MALDI MS. In preliminary experiments a peptide corresponding to a C-terminal fragment of ArsA had the additional mass of the derivative.